U.S. patent application number 16/198286 was filed with the patent office on 2019-05-30 for iridium oxide electrodeposited porous titanium composite layer of polymer electrolyte membrane water electrolysis apparatus, met.
This patent application is currently assigned to KOREA INSTITUTE OF SCIENCE AND TECHNOLOGY. The applicant listed for this patent is KOREA INSTITUTE OF SCIENCE AND TECHNOLOGY. Invention is credited to Seunghoe CHOE, Dirk HENKENSMEIER, Jong Hyun JANG, Hyoung-Juhn KIM, Jin Young KIM, Hye Jin LEE, So Young LEE, Ahyoun LIM, Youngseung NA, Hyun Seo PARK, Sung Jong YOO.
Application Number | 20190161868 16/198286 |
Document ID | / |
Family ID | 66634939 |
Filed Date | 2019-05-30 |
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United States Patent
Application |
20190161868 |
Kind Code |
A1 |
JANG; Jong Hyun ; et
al. |
May 30, 2019 |
IRIDIUM OXIDE ELECTRODEPOSITED POROUS TITANIUM COMPOSITE LAYER OF
POLYMER ELECTROLYTE MEMBRANE WATER ELECTROLYSIS APPARATUS, METHOD
FOR PREPARING THE SAME, AND POLYMER ELECTROLYTE MEMBRANE WATER
ELECTROLYSIS APPARATUS USING THE SAME
Abstract
The present disclosure relates to an IrO.sub.2 electrodeposited
porous titanium composite layer of a polymer electrolyte membrane
water electrolysis apparatus serving as both a diffusion layer and
an oxygen electrode, the apparatus including: a porous titanium
(Ti) layer; and an electrodeposited iridium oxide (IrO.sub.2) layer
on the porous Ti layer. The IrO.sub.2 layer may be uniformly
deposited on a porous Ti layer through an electrolysis process, and
the electrodeposited IrO.sub.2 layer may play multiple roles as not
only a catalyst layer toward oxygen evolution reaction (OER) on the
surface of the Ti layer, but also a corrosion-protection layer
which prevents an inner Ti layer from corrosion.
Inventors: |
JANG; Jong Hyun; (Seoul,
KR) ; CHOE; Seunghoe; (Seoul, KR) ; NA;
Youngseung; (Seoul, KR) ; LEE; Hye Jin;
(Seoul, KR) ; LIM; Ahyoun; (Seoul, KR) ;
KIM; Hyoung-Juhn; (Seoul, KR) ; HENKENSMEIER;
Dirk; (Seoul, KR) ; YOO; Sung Jong; (Seoul,
KR) ; KIM; Jin Young; (Seoul, KR) ; LEE; So
Young; (Seoul, KR) ; PARK; Hyun Seo; (Seoul,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KOREA INSTITUTE OF SCIENCE AND TECHNOLOGY |
Seoul |
|
KR |
|
|
Assignee: |
KOREA INSTITUTE OF SCIENCE AND
TECHNOLOGY
Seoul
KR
|
Family ID: |
66634939 |
Appl. No.: |
16/198286 |
Filed: |
November 21, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 11/0452 20130101;
C25B 9/10 20130101; C25B 11/0447 20130101; C25B 1/10 20130101; C25D
9/04 20130101; C25B 11/0415 20130101; C25D 9/06 20130101; C25B
11/035 20130101 |
International
Class: |
C25B 1/10 20060101
C25B001/10; C25B 11/03 20060101 C25B011/03; C25B 11/04 20060101
C25B011/04; C25D 9/04 20060101 C25D009/04 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2017 |
KR |
10-2017-0161523 |
Claims
1. An IrO.sub.2 electrodeposited porous titanium composite layer of
a polymer electrolyte membrane water electrolysis apparatus serving
as both a diffusion layer and an oxygen electrode, the apparatus
comprising: a porous titanium (Ti) layer; and an electrodeposited
iridium oxide (IrO.sub.2) layer on the porous Ti layer.
2. The composite layer of the polymer electrolyte membrane water
electrolysis apparatus according to claim 1, wherein the porous Ti
layer comprises one or more selected from a group consisting of Ti
layers in a form of titanium paper and mesh.
3. The composite layer of the polymer electrolyte membrane water
electrolysis apparatus according to claim 1, wherein the IrO.sub.2
layer comprises iridium oxide loaded at 0.01 to 1.05 mg/cm.sup.2
onto the porous Ti layer.
4. The composite layer of the polymer electrolyte membrane water
electrolysis apparatus according to claim 1, wherein the IrO.sub.2
layer comprises iridium oxide loaded at 0.1 to 0.5 mg/cm.sup.2 onto
the porous Ti layer.
5. The composite layer of the polymer electrolyte membrane water
electrolysis apparatus according to claim 1, wherein the IrO.sub.2
layer is uniformly deposited on the porous Ti layer, and thus
physically blocks the porous Ti layer from the external oxidative
environment.
6. The composite layer of the polymer electrolyte membrane water
electrolysis apparatus according to claim 1, wherein the IrO.sub.2
layer has a thickness of 10 to 210 nm.
7. The composite layer of the polymer electrolyte membrane water
electrolysis apparatus according to claim 1, wherein the IrO.sub.2
layer comprises amorphous iridium oxide.
8. A method for preparing an IrO.sub.2 electrodeposited porous
titanium composite layer of a polymer electrolyte membrane water
electrolysis apparatus serving as both a diffusion layer and an
oxygen electrode, the method comprising: electrodepositing an
iridium oxide (IrO.sub.2) layer on a porous titanium (Ti)
layer.
9. The method according to claim 8, wherein the porous Ti layer
comprises one or more selected from a group consisting of Ti layers
in a form of titanium paper and mesh.
10. The method according to claim 8, wherein the electrodeposition
is performed at a deposition potential of 0.5 to 0.9 V.sub.SCE.
11. The method according to claim 8, wherein the electrodeposition
is performed for 1 min or more and less than 10 min.
12. The method according to claim 8, wherein the electrodeposition
is performed for 3 min or more and 8 min or less.
13. A polymer electrolyte membrane water electrolysis apparatus
comprising the composite layer of the polymer electrolyte membrane
water electrolysis apparatus according to claim 1.
14. The polymer electrolyte membrane water electrolysis apparatus
according to claim 13, wherein the polymer electrolyte membrane
water electrolysis apparatus exhibits a current density of 0.07 to
0.96 A/cm.sup.2 at 1.6 V and 120.degree. C.
15. The polymer electrolyte membrane water electrolysis apparatus
according to claim 13, wherein the polymer electrolyte membrane
water electrolysis apparatus exhibits a current density of 0.16 to
0.96 A/cm.sup.2.
16. The polymer electrolyte membrane water electrolysis apparatus
according to claim 13, wherein the polymer electrolyte membrane
water electrolysis apparatus is a high-temperature polymer
electrolyte membrane water electrolysis apparatus which is operated
at 200.degree. C. or less.
17. The polymer electrolyte membrane water electrolysis apparatus
according to claim 16, wherein the polymer electrolyte membrane
water electrolysis apparatus is a high-temperature polymer
electrolyte membrane water electrolysis apparatus which is operated
at 60.degree. C. or more.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority of Korean Patent
Application No. 10-2017-0161523, filed on Nov. 29, 2017, and all
the benefits accruing therefrom under 35 U.S.C. .sctn. 119, the
contents of which in its entirety are herein incorporated by
reference.
BACKGROUND
1. Field
[0002] The present disclosure relates to an IrO.sub.2
electrodeposited porous titanium composite layer for a polymer
electrolyte membrane water electrolysis (PEMWE), a method for
preparing the same, and a polymer electrolyte membrane water
electrolysis apparatus using the same.
[0003] [Description of the National Support Research and
Development]
[0004] This study was conducted with the support from the National
Research Foundation of Korea funded by the Ministry of Science and
ICT under the supervision of the Korea Institute of Energy
Research, the project title is the KOREA CCS 2020 Project, and the
project name is `Development of innovation technology for CO.sub.2
conversion to syn-gas by low temperature electrolysis`
(2N52570)(Project Identification No. 1711057994).
[0005] This study was conducted with the support from the National
Research Foundation of Korea funded by the Ministry of Science and
ICT under the supervision of the Korea Institute of Energy
Research, the project title is the development of climate change
mitigation technology, and the project name is `Development of
innovative technologies for low-cost and durable low-Pt
electrocatalysts and electrodes for PEMFC MEAs (2N52950)(Project
Identification No. 1711060115).
2. Description of the Related Art
[0006] Hydrogen is rapidly emerging as an alternative as a future
energy carrier for overcoming issues related to environment and
coal exhaustion. However, currently, the production of hydrogen is
mainly dependent on petrochemical processes such as steam-reforming
of natural gas (48%), partial oxidation (30%), and coal
gasification (18%), and inevitably emits carbon into atmosphere.
Water electrolysis, a technology to decompose water into hydrogen
and oxygen gases using electricity, may produce clean hydrogen with
zero- or low-carbon footprint.
[0007] Currently, the alkaline water electrolysis (AWE) has
occupied most of electrolyzer market, but there are drawbacks in
that the purity of hydrogen produced is relatively low, there is a
need for large-scale plant, and toxic solvents are inevitably
used.
[0008] As an alternative, the polymer electrolyte membrane water
electrolysis (PEMWE) has received attention particularly for
small-scale hydrogen production (<300 kW) and surplus power
storage because the PEMWE may produce hydrogen having excellent
purity at a high yield. Besides, the PEMWE has been widely used
because there are advantages in that the system has a small volume
and harmful materials are not emitted, and the R&D know how
from its sister technology, polymer electrolyte fuel cell (PEMFC),
has also provided significant insight for further development of
the PEMWE.
[0009] However, the use of the PEMWE has a drawback in that the
price of produced hydrogen (4.1 to 8.6 /kgH.sub.2) is relatively
high compared to that of the AWE (3.2 to 5.9 /kgH.sub.2), and the
high price is mainly ascribed by electricity cost (1.2 to 4.0
/kgH.sub.2), stack replacement cost (0.86 to 2.8 /kgH.sub.2), and
capital cost (0.85 to 2.9 /kgH.sub.2). The unit cell of
conventional PEMWE is composed of a Ti bipolar plate/a Ti-based
diffusion layer/an IrO.sub.2 catalyst (1 to 5 mg/cm.sup.2)/a nafion
membrane/a Pt catalyst (0.5 to 1 mg/cm.sup.2)/a carbon-based
diffusion layer/a graphite bipolar plate. The corresponding unit
cell is generally operated under a temperature condition of 60 to
90.degree. C. and an atmosphere pressure condition, and at this
time, the cell voltage exhibits, for example, 1.55 to 1.8 V at 1
A/cm.sup.2.
[0010] Meanwhile, in order to reduce the hydrogen production cost
by the PEMWE, studies have been continuously conducted to lower
voltage loss for reducing electricity cost and improve cell
durability for reducing stack replacement cost, and studies for
reducing capital & stack replacement cost have been
continuously conducted.
[0011] Among them, lowering voltage loss can be accomplished by
boosting oxygen evolution reaction (OER) at anode, which is a major
contributor for voltage loss, by developing efficient catalyst and
membrane electrolyte, and the cell durability issues can be
resolved by using a coated (Au, Pt) bipolar plate or a stable
membrane.
[0012] Meanwhile, the high temperature polymer electrolyte membrane
water electrolysis (HT-PEMWE), which is operated at a high
temperature range (100.degree. C..ltoreq.T.ltoreq.200.degree. C.),
may be expected to provide better cell efficiency and lower
electricity cost than conventional PEMWE. Among them, the better
cell efficiency is associated with fast electrode reaction due to
better electrode kinetics and lower equilibrium voltage, and for
example, when the temperature is raised from 80 to 120.degree. C.,
the rate constant is raised up at least 4-folds because the
activation energy for oxygen evolution reaction on an IrO.sub.2
catalyst is 40 to 58 kJ/mol. Further, the Nernst voltage for water
spitting is decreased by 33 mV.
[0013] When a cell is operated at the high temperature, a membrane
dehydration problem may occur, and the membrane dehydration problem
may drop proton conductivity and increase ohmic loss. Furthermore,
the high temperature PEMWE has a relatively inferior cell stability
resulting from high operating temperature, which may inevitably
increase the stack replacement cost, and the increase in stack
replacement cost is because among the Ti-based anode components,
which have been widely applied for conventional PEMWE, a diffusion
layer (DL), a bipolar plate (BP), and a nafion membrane are
unstable at operation conditions of HT-PEMWE, and at high
temperature, Ti is rapidly oxidized (FIG. 1A) and corroded at
anodic condition (>1.0 V.sub.SHE), and this process degrades the
quality of the DL and BP. For this reason, there have been attempts
to apply a protective coating such as Au, Pt, and Ta to the
diffusion layer and the bipolar plate, but these attempts
inevitably increase the initial capital cost (FIG. 1B). Further,
even though the Ti bipolar plate or diffusion layer is protected by
Pt, corrosion may occur when the Ti layer is not completely
covered. Therefore, there is a need for developing a material
capable of solving the stability issue for PEMWE as a stable and
cost-efficient material for a diffusion layer, a bipolar plate, and
a membrane electrolyte.
REFERENCES OF THE RELATED ART
Non-Patent Documents
[0014] (Non-Patent Document 1) B.-S. Lee, H.-Y. Park, I. Choi, M.
K. Cho, H.-J. Kim, S. J. Yoo, D. Henkensmeier, J. Y. Kim, S. W.
Nam, S. Park, K.-Y. Lee, J. H. Jang, J. Power Sources 309 (2016)
127-134. [0015] (Non-Patent Document 2) B.-S. Lee, S. H. Ahn, H.-Y.
Park, I. Choi, S. J. Yoo, H.-J. Kim, D. Henkensmeier, J. Y. Kim, S.
Park, S. W. Nam, K.-Y. Lee, J. H. Jang, Appl. Catal. B., 179 (2015)
285-291. [0016] (Non-Patent Document 3) A. Skulimowska, M. Dupont,
M. Zaton, S. Sunde, L. Merlo, D. J. Jones, J. Roziere, Int. J.
Hydrogen Energy 39 (2014) 6307-6316. [0017] (Non-Patent Document 4)
V. Antonucci, A. Di Blasi, V. Baglio, R. Ornelas, F. Matteucci, J.
Ledesma-Garcia, L. G. Arriaga, A. S. Arico, Electrochim. Acta 53
(2008) 7350-7356. [0018] (Non-Patent Document 5) V. Baglio, R.
Ornelas, F. Matteucci, F. Martina, G. Ciccarella, I. Zama, L. G.
Arriaga, V. Antonucci, A. S. Arico, Fuel Cells 9 (2009) 247-252.
[0019] (Non-Patent Document 6) D. Aili, M. K. Hansen, C. Pan, Q.
Li, E. Christensen, J. O. Jensen, N. J. Bjerrum, Int. J. Hydrogen
Energy 36 (2011) 6985-6993. [0020] (Non-Patent Document 7) W. Xu,
K. Scott, S. Basu, J. Power Sources 196 (2011) 8918-8924. [0021]
(Non-Patent Document 8) M. K. Hansen, D. Aili, E. Christensen, C.
Pan, S. Eriksen, J. O. Jensen, J. H. von Barner, Q. Li, N. J.
Bjerrum, Int. J. Hydrogen Energy 37 (2012) 10992-11000. [0022]
(Non-Patent Document 9) P. Mazur, J. Polonsky, M. Paidar, K.
Bouzek, Int. J. Hydrogen Energy 37 (2012) 12081-12088. [0023]
(Non-Patent Document 10) V. Natarajan, S. Basu, K. Scott, Int. J.
Hydrogen Energy 38 (2013) 16623-16630. [0024] (Non-Patent Document
11) H. Li, T. Fujigaya, H. Nakajima, A. Inada, K. Ito, J. Power
Sources 332 (2016) 16-23. [0025] (Non-Patent Document 12) H. Li, A.
Inada, T. Fujigaya, H. Nakajima, K. Sasaki, K. Ito, J. Power
Sources 318 (2016) 192-199. [0026] (Non-Patent Document 13) J.
Mali, P. Mazur, M. Paidar, T. Bystron, K. Bouzek, Int. J. Hydrogen
Energy 41 (2016) 2177-2188.
SUMMARY
[0027] In an aspect, the present disclosure is directed to
providing an IrO.sub.2 electrodeposited porous titanium composite
layer of a polymer electrolyte membrane water electrolysis
apparatus. In the composite layer serving as both a diffusion layer
and an oxygen electrode, the electrodeposited IrO.sub.2 film plays
a dual role of a catalyst layer for the oxygen evolution reaction
(OER) and a corrosion-protection layer that prevents oxidation of
the inner Titanium.
[0028] In another aspect, the present disclosure is directed to an
apparatus having low IrO.sub.2 loading amount and high
stability.
[0029] In further another aspect, the present disclosure is
directed to providing a polymer electrolyte membrane water
electrolysis apparatus including the composite layer of the polymer
electrolyte membrane water electrolysis apparatus.
[0030] In embodiments, the present disclosure provides an IrO.sub.2
electrodeposited porous titanium composite layer of a polymer
electrolyte membrane water electrolysis apparatus serving as both a
diffusion layer and an oxygen electrode, the apparatus including: a
porous titanium (Ti) layer; and an electrodeposited iridium oxide
(IrO.sub.2) layer on the porous Ti layer.
[0031] In an exemplary embodiment, the porous Ti layer may include
one or more selected from a group consisting of Ti layers in a form
of titanium paper and mesh.
[0032] In another exemplary embodiment, the IrO.sub.2 layer may
include iridium oxide loaded at 0.01 to 1.05 mg/cm.sup.2 onto the
porous Ti layer.
[0033] In another exemplary embodiment, the IrO.sub.2 layer may
include iridium oxide loaded at 0.1 to 0.5 mg/cm.sup.2 onto the
porous Ti layer.
[0034] In another exemplary embodiment, the IrO.sub.2 layer may be
uniformly deposited on the porous Ti layer, and thus may physically
block the porous Ti layer from the external oxidative
environment.
[0035] In another exemplary embodiment, the IrO.sub.2 layer may
have a thickness of 10 to 210 nm.
[0036] In another exemplary embodiment, the IrO.sub.2 layer may
include amorphous iridium oxide.
[0037] In another aspect, the present disclosure provides a method
for preparing an IrO.sub.2 electrodeposited porous titanium
composite layer of a polymer electrolyte membrane water
electrolysis apparatus serving as both a diffusion layer and an
oxygen electrode, the method including: electrodepositing an
iridium oxide (IrO.sub.2) layer on a porous titanium (Ti)
layer.
[0038] In an exemplary embodiment, the porous Ti layer may include
one or more selected from a group consisting of Ti layers in a form
of titanium paper and mesh.
[0039] In another exemplary embodiment, the electrodeposition may
be performed at a deposition potential of 0.5 to 0.9 V.sub.SCE.
[0040] In another exemplary embodiment, the electrodeposition may
be performed for 1 min or more and less than 10 min.
[0041] In another exemplary embodiment, the electrodeposition may
be performed for 3 min or more and 8 min or less.
[0042] In still another aspect, the present disclosure provides a
polymer electrolyte membrane water electrolysis apparatus including
the IrO.sub.2 electrodeposited porous titanium composite layer of
the polymer electrolyte membrane water electrolysis apparatus.
[0043] In an exemplary embodiment, the polymer electrolyte membrane
water electrolysis apparatus may exhibit a current density of 0.07
to 0.96 A/cm.sup.2 at 1.6 V and 120.degree. C.
[0044] In an exemplary embodiment, the polymer electrolyte membrane
water electrolysis apparatus may exhibit a current density of 0.16
to 0.96 A/cm.sup.2.
[0045] In an exemplary embodiment, the polymer electrolyte membrane
water electrolysis apparatus may be a high-temperature polymer
electrolyte membrane water electrolysis apparatus which is operated
at 200.degree. C. or less.
[0046] In an exemplary embodiment, the polymer electrolyte membrane
water electrolysis apparatus may be a high-temperature polymer
electrolyte membrane water electrolysis apparatus which is operated
at 60.degree. C. or more.
[0047] A composite layer of a polymer electrolyte membrane water
electrolysis apparatus according to embodiments of the present
disclosure may include an electrodeposited iridium oxide
(IrO.sub.2) layer on a porous titanium (Ti) layer. Herein, the
electrodeposited IrO.sub.2 layer physically blocks Ti on the Ti
layer, and thus may contribute to the prevention of a porous Ti
layer from corrosion, and may also function as a catalyst for an
oxygen evolution reaction (OER). In the present disclosure, a
porous Ti layer, more specifically, a Ti layer in a form of
titanium paper or mesh is used, and in this case, iridium oxide may
be more uniformly deposited, and thus may more effectively prevent
the Ti layer from corrosion. Accordingly, the performance of a
polymer electrolyte membrane water electrolysis apparatus including
the same may be significantly increased, and particularly, the
anode may exhibit high performance (for example, 0.97 A/cm.sup.2 at
1.6 V and 120.degree. C.) despite low IrO.sub.2 loading amount (0.4
mg/cm.sup.2), which is comparable to conventional anodes (when a
catalyst is loaded at 0.8 to 5 mg/cm.sup.3).
[0048] Furthermore, the composite layer of the polymer electrolyte
membrane water electrolysis apparatus according to embodiments of
the present disclosure may exhibit excellent durability even at
120.degree. C. and 1.72 V (voltage efficiency: 85%) when applied to
a cell, so that the composite layer may be applied to a polymer
electrolyte membrane water electrolysis apparatus which is operated
at a high temperature of, for example, 200.degree. C. or less.
Therefore, the composite layer of the polymer electrolyte membrane
water electrolysis apparatus may be used without corrosion for a
long period of time by using a high-temperature polymer electrolyte
membrane water electrolysis apparatus.
[0049] In summary, the composite layer according to embodiments of
the present disclosure may exhibit high performance despite low
IrO.sub.2 loading amount, which is comparable to conventional
composite layers, and has excellent durability, and thus may be
widely used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] FIGS. 1A and 1B show an anode for a polymer electrolyte
membrane water electrolysis apparatus according to the related
art.
[0051] FIG. 2 shows a diffusion layer and an oxygen electrode
composite layer for a polymer electrolyte membrane water
electrolysis apparatus according to the present disclosure.
[0052] FIG. 3 shows the current profile during IrO.sub.2
electrodeposition under E.sub.dep conditions of 0.5, 0.6, 0.7, 0.8,
and 0.9 V.sub.SCE.
[0053] FIGS. 4A to 4E show the surfaces of pristine Ti mesh and
iridium oxide-deposited Ti mesh (e-TiO.sub.2/Ti), respectively
(FIG. 4A: pristine Ti mesh, FIGS. 4B to 4E: iridium oxide-deposited
Ti mesh under conditions of 0.6 V.sub.SCE (FIG. 4B), 0.7 V.sub.SCE
(FIG. 4C), 0.8 V.sub.SCE (FIG. 4D), and 0.9 V.sub.SCE (FIG. 4E) for
5 min). The corresponding EPMA mapping results for Ir are also
shown below, respectively.
[0054] FIG. 5 is XPS spectra for e-IrO.sub.2/Ti deposited under a
condition of 0.7 V.sub.SCE for 5 min.
[0055] FIGS. 6A to 6E show surface changes during the preparation
of e-IrO.sub.2/Ti when time conditions are varied. At this time,
t.sub.dep was controlled to 1 min (FIG. 6A), 3 min (FIG. 6B), 5 min
(FIG. 6C), 10 min (FIG. 6D), and 20 min(FIG. 6E), and at this time,
E.sub.dep was fixed at 0.7 V.sub.SCE.
[0056] FIG. 7A is a graph showing IrO.sub.2 loading amount as a
function of t.sub.dep under the E.sub.dep condition fixed at 0.7
V.sub.SCE, and FIG. 7B is a photograph showing the cross-section of
e-IrO.sub.2/Ti at E.sub.dep of 0.7 V and t.sub.dep of 5 min.
[0057] FIG. 8A is iV curves for MEA using e-IrO.sub.2/Ti produced
by varying t.sub.dep under the E.sub.dep condition fixed at 0.7
V.sub.SCE, and FIG. 8B shows ohmic overpotential (.eta..sub.ohm)
and mass transfer overpotential (.eta..sub.mass) values at 0.75
A/cm.sup.2 for e-IrO.sub.2/Ti (t of 5, 10, and 20 min).
[0058] FIGS. 9A and 9B show changes in (A) current density and (B)
mass activity according to the change in catalyst loading amount at
1.6 V, 120.degree. C., and 2.5 bar in an e-IrO.sub.2/Ti
(E.sub.dep=0.7 V) electrode. At this time, values of other cells
within an operating temperature range from 110 to 130.degree. C.
are also presented.
[0059] FIG. 10 shows polarization curves for initial and
IrO.sub.2-electrodeposited Ti meshes in 0.5 M H.sub.2SO.sub.4 at
25.degree. C. (Inset) at a critical current density as a function
of t.sub.dep.
[0060] FIG. 11 is a graph showing operation performance drop
experimental results of single cells including e-IrO.sub.2/Ti,
e-IrO.sub.2/CP, and s-IrO.sub.2/Ti electrodes under the conditions
of 1.72 V, 120.degree. C., and 2.5 bar.
DETAILED DESCRIPTION
[0061] Hereinafter, embodiments of the present disclosure will be
described in detail with reference to accompanying drawings.
Embodiments of the present disclosure have been described with
reference to accompanying drawings, but have been described for
illustration, and the technical spirit of the present disclosure
and the configuration and application thereof are not limited
thereby.
DEFINITION OF TERMS
[0062] In the present specification, the term `titanium layer in a
form of mesh` refers to a porous titanium layer having a shape such
as a network. The network pore may have, for example, a polygonal
or circular structure.
[0063] In the present specification, the term `titanium paper`
refers to a substrate in the form of paper formed by closely
weaving cylindrical or polygonal drum-like titanium (Ti) fiber. The
titanium fiber may have a cross-sectional area of 50 to 10,000
.mu.m.sup.2.
[0064] IrO.sub.2 Electrodeposited Porous Titanium Composite Layer
of Polymer Electrolyte Membrane Water Electrolysis Apparatus
[0065] The present disclosure provides an IrO.sub.2
electrodeposited porous titanium composite layer of a polymer
electrolyte membrane water electrolysis apparatus serving as both a
diffusion layer and an oxygen electrode, the apparatus including: a
porous titanium (Ti) layer; and an electrodeposited iridium oxide
(IrO.sub.2) layer on the porous Ti layer. In the composite layer,
an electrodeposited IrO.sub.2 layer may function as a
corrosion-protection layer which prevents an inner Ti layer from
corrosion and a catalyst layer toward OER. Furthermore, the
electrodeposited IrO.sub.2 layer may function as an electrode layer
along with the Ti layer. Accordingly, the inner porous Ti layer may
not be corroded from anodic oxidation, and thus, may exhibit
excellent durability even when the composite layer is used for a
high-temperature polymer electrolyte membrane water electrolysis
apparatus which is operated at high temperature (for example, about
200.degree. C. or less). Furthermore, the IrO.sub.2 is coated on
the Ti layer through an electrodeposition process, and in this
case, the IrO.sub.2 may exhibit excellent effect even though the
loading amount is small (that is, the mass activity is excellent).
Accordingly, the durability of Ti may be more improved, and the OER
may be more effectively increased by a small IrO.sub.2 loading
amount.
[0066] In an exemplary embodiment, a porous Ti layer may be used
because titanium exhibits strong durability under high-temperature
polymer water electrolysis conditions due to high oxidation
resistance and corrosion resistance as compared to a material such
as carbon or copper which is a conductive material, and is
competitive in terms of price as compared to gold, platinum, and
the like, and thus, the porous Ti layer is preferred. In addition,
when the porous Ti layer is used, IrO.sub.2 may be more smoothly
coated.
[0067] Specifically, the porous Ti layer may include a plurality of
pores. In an aspect, the porous Ti layer may have a porosity of
about 50% to about 80%, preferably about 60% to about 70%.
[0068] In an exemplary embodiment, the porous Ti layer may include
one or more selected from a group consisting of Ti layers in a form
of titanium paper and mesh.
[0069] In an aspect, the Ti layer may be a Ti layer in the form of
mesh, and when the Ti layer has a mesh shape as described above, a
deposition process may be uniformly performed as compared to
titanium paper, and the like, during the deposition of the
IrO.sub.2 layer on the Ti layer. Accordingly, an effect such as
prevention of the Ti layer from corrosion may be more enhanced.
[0070] In an exemplary embodiment, the Ti layer may have a
thickness of 50 nm to 1,000 nm, and may have a thickness of
specifically 100 nm to 500 nm. When the Ti layer has a thickness of
less than 50 nm, physical stability may deteriorate, and when the
Ti layer has a thickness of more than 1,000 nm, performance drop
per catalyst amount may occur due to problems such as reduction in
mass transfer at anode and an increase in amount of catalyst
(IrO.sub.2) required.
[0071] In an exemplary embodiment, the Ti layer may have an active
area of 1 to 100 cm.sup.2.
[0072] Meanwhile, in the present disclosure, the IrO.sub.2 layer is
a coating layer which is coated on the Ti layer, and may function
as an electrode layer of oxygen reaction.
[0073] In an exemplary embodiment, the IrO.sub.2 layer is
electrodeposited on the Ti layer, and when the IrO.sub.2 layer is
electrodeposited, a dense coating thin film is formed on the Ti
surface unlike the case when other preparation methods for loading
an IrO.sub.2 catalyst (for example, a spraying method or a decal
method, and the like) are used on the corresponding Ti layer. The
IrO.sub.2 coating layer may also serve as an oxidation prevention
film which prevents the inner Ti layer from corrosion unlike the
case of using other conventional methods, which use only IrO.sub.2
as a catalyst.
[0074] In an aspect, a diffusion layer and an oxygen electrode
composite layer of an electrolyte membrane water electrolysis
apparatus may be expressed as a structure in which the surface of
the porous Ti layer is covered with an IrO.sub.2 layer and
IrO.sub.2 particles are deposited onto the corresponding IrO.sub.2
layer.
[0075] In an exemplary embodiment, the IrO.sub.2 layer covers the
surface of the porous Ti layer, and it is preferred that the
IrO.sub.2 layer has a crack-free structure.
[0076] That is, a preferred composite layer structure is a
crack-free structure in which the IrO.sub.2 layer covers the
surface of the porous Ti layer, and the IrO.sub.2 layer does not
have cracks, and a more preferred composite layer structure is a
crack-free structure in which the IrO.sub.2 layer covers the
surface of a Ti layer in the form of mesh, and the IrO.sub.2 layer
does not have cracks.
[0077] In another exemplary embodiment, the IrO.sub.2 layer may
include amorphous iridium oxide. In this case, the iridium oxide
included in the IrO.sub.2 layer may exhibit a density of 10
g/cm.sup.3 or less, and specifically 7 to 9 g/cm.sup.3.
[0078] In another exemplary embodiment, the IrO.sub.2 layer may
have a thickness of 10 to 210 nm, preferably 140 to 210 nm. When
the IrO.sub.2 layer has a thickness of less than 10 nm, the inner
titanium corrosion prevention capability may be significantly
decreased, and when the IrO.sub.2 layer has a thickness of more
than 210 nm, cracks are generated inside thereof, and as a result,
the corrosion prevention capability may be decreased.
[0079] Meanwhile, in the present disclosure, the IrO.sub.2 layer
may exhibit an effect which is the same as or more improved than
that of a conventional anode even though a small amount of
IrO.sub.2 is loaded onto the Ti layer. For example, even though
IrO.sub.2 at 0.01 to 1.05 mg/cm.sup.2, specifically, 0.1 to 0.5
mg/cm.sup.2 is loaded onto the Ti layer, it is possible to exhibit
an effect which is the same as or more improved than that of an
anode of a conventional polymer electrolyte membrane water
electrolysis apparatus.
[0080] Meanwhile, in another aspect, the present disclosure
provides a method for preparing an IrO.sub.2 electrodeposited
porous Titanium composite layer of a polymer electrolyte membrane
water electrolysis apparatus serving as both the diffusion layer
and an oxygen electrode, the method including: electrodepositing an
iridium oxide (IrO.sub.2) layer on a porous titanium (Ti)
layer.
[0081] In an exemplary embodiment, the electrodeposition may be
proceeded at a deposition potential of 0.5 to 0.9 V.sub.SCE, more
specifically, 0.6 to 0.9 V.sub.SCE. When the electrodeposition is
performed at less than 0.5 V.sub.SCE, a non-uniform coating layer
may be formed because the deposition rate is very slow and the
deposition is non-uniformly performed, and when the
electrodeposition is performed at more than 0.9 V, the deposition
efficiency is not excellent, and as a result, the range may not be
preferred in terms of economic efficiency.
[0082] In an exemplary embodiment, the electrodeposition may be
performed for 1 min or more and less than 10 min, preferably, 3 min
or more and 8 min or less. When the electrodeposition is performed
for less than 1 min, the IrO.sub.2 layer may not completely cover
the Ti layer, and when the electrodeposition is performed for more
than 10 min, cracks may be produced on the IrO.sub.2 layer.
[0083] Meanwhile, in another aspect, the present disclosure
provides a polymer electrolyte membrane water electrolysis
apparatus including a polymer electrolyte membrane, a cathode
formed at one side of the membrane, and an anode formed at the
other side of the membrane, in which the anode includes the
composite layer.
[0084] In an exemplary embodiment, a membrane electrode assembly is
prepared by forming a composite layer including a porous Ti layer
in which the IrO.sub.2 layer is electrodeposited at one side of a
polymer electrolyte membrane (for example, a nafion membrane), and
forming a cathode (a commercial cathode may be used, and for
example, Pt/C formed by spraying platinum onto carbon paper may be
used) at the other side thereof, and thus, a polymer electrolyte
membrane water electrolysis apparatus (cell) may be prepared by
assembling a bipolar plate with the membrane electrode assembly and
mounting an end plate thereto.
[0085] In an exemplary embodiment, the polymer electrolyte membrane
water electrolysis apparatus may achieve high performance even in a
small catalyst amount (small IrO.sub.2 loading amount). For
example, when the IrO.sub.2 loading amount is 0.01 to 1.05
mg/cm.sup.2, it is possible to exhibit a current density of 0.07 A
cm.sup.-2 to 0.96 A cm.sup.-2 at 1.6 V, 120.degree. C., and 1.0 to
700 bar (or, for example, 1.0 to 50 bar, or 1.0 to 30 bar, 1.0 to
2.5 bar), and in this case, as the cathode and the membrane, it is
possible to use a typical cathode (for example, Pt/C: the cathode
catalyst amount may also be a typical catalyst amount, for example,
0.4 mg/cm.sup.2) and a typical membrane (nafion).
[0086] In an aspect, when the IrO.sub.2 loading amount is 0.1 to
0.5 mg/cm.sup.2, the polymer electrolyte membrane water
electrolysis apparatus may exhibit a current density of 0.16 to
0.96 A cm.sup.-2 under the same conditions as described above.
[0087] Meanwhile, the polymer electrolyte membrane water
electrolysis apparatus showed stability under a high-temperature
water electrolysis condition (200.degree. C. or less), and showed a
degradation rate of about 0.1%/hr to about 0.2%/hr under the
conditions of, for example, 1 V to 3 V, 100.degree. C. to
120.degree. C., and 1 bar to 3 bar.
[0088] In an exemplary embodiment, it could be confirmed that the
polymer electrolyte membrane water electrolysis apparatus showed a
degradation rate of about 0.11%/hr at 1.72 V, 120.degree. C., and
2.5 bar.
[0089] As described above, the features of aspects of the present
disclosure, particularly, characteristics for the electrode
stability may be clearly exhibited at high temperature (a
temperature of 200.degree. C. or less). The reason is because among
the Ti-based anode components which are widely applied to
conventional PEMWE, the diffusion layer (DL), the bipolar plate
(BP), and the nafion membrane are unstable under the operating
conditions of the high-temperature polymer electrolyte membrane
water electrolysis apparatus (HT-PEMWE), Ti is more rapidly
oxidized particularly at high temperature (T.ltoreq.200.degree. C.)
and thus is corroded under anodic conditions, and the procedure
degrades the quality of DL and BP. In contrast, according to the
present disclosure, since an electrodeposited IrO.sub.2 layer,
which serves both as a catalyst layer and a corrosion-protection
layer in a composite layer, perfectly physically blocks a porous Ti
layer, the oxidation of the Ti layer may be reduced, thereby
solving a problem in that the performance of the electrode
deteriorates. Accordingly, a high-temperature polymer electrolyte
membrane water electrolysis apparatus may be stably operated for a
long period of time, and hydrogen may be produced with high
efficiency.
[0090] In an exemplary embodiment, a polymer electrolyte membrane
water electrolysis apparatus including the composite layer may be a
high-temperature polymer electrolyte membrane water electrolysis
apparatus which is implemented at a temperature of, specifically,
200.degree. C. or less, specifically, at a temperature of 60 to
200.degree. C.
[0091] In an aspect, a polymer electrolyte membrane water
electrolysis apparatus including the composite layer may be
implemented at a temperature of 60 to 150.degree. C., or at a
temperature of 100 to 150.degree. C.
[0092] In an aspect, a polymer electrolyte membrane water
electrolysis apparatus including the composite layer may be
implemented at a temperature of 60 to 120.degree. C.
[0093] Further, in another aspect, the present disclosure provides
a polymer electrolyte membrane water electrolysis method using the
polymer electrolyte membrane water electrolysis apparatus.
[0094] Hereinafter, the present disclosure will be described in
more detail through Examples. These Examples are only for
exemplifying the present disclosure, and it will be obvious to
those skilled in the art that the scope of the present disclosure
is not interpreted to be limited by these Examples.
Examples
[0095] Hereinafter, the present disclosure will be described in
detail through an example and test examples. However, the following
example and test examples are for illustrative purposes only and it
will be apparent to those of ordinary skill in the art that the
scope of the present disclosure is not limited by them.
[0096] (1) Preparation of Anode
[0097] An oxygen electrode (anode) was prepared by performing
electrodeposition or spraying processes on two kinds of diffusion
layers (Ti and carbon paper (CP)). A solution consisting of 10 mM
iridium chloride hydrate (IrCl.sub.4.H.sub.2O), 100 mM hydrogen
peroxide H.sub.2O.sub.2, 40 mM oxalic acid (COOH.sub.2.2H.sub.2O),
and 340 mM potassium carbonate (KCO.sub.3) was prepared as an
electrodeposition solution.
[0098] First, a Ti mesh (Ti gauze 80 mesh, alpha aeaser) substrate
was preliminary immersed into 5 wt % oxalic acid (60.degree. C.)
for 30 min for removal of native oxide (TiO.sub.2). On the Ti mesh
substrate, an e-IrO.sub.2/Ti electrode was prepared via
electrodeposition with various deposition potentials (E.sub.dep,
0.5 to 0.8 V.sub.SCE) and deposition times (t.sub.dep, 1 to 20 min)
at room temperature. At this time, a Ti mesh with an active area of
32 cm.sup.2 and a standard calomel electrode (SCE) were employed as
counter and reference electrodes, respectively.
[0099] For comparison, the e-IrO.sub.2/CP electrode was prepared by
electrodepositing IrO.sub.2 (E.sub.dep=0.7 V and t.sub.dep=10 min)
on carbon paper (TGPH-090, Toray) without any pretreatment. All
processes were carried out with potentiostat equipment (AUT302N,
AUTO LAB Ltd.).
[0100] Meanwhile, an s-IrO.sub.2/Ti electrode was prepared by
spraying a catalyst ink (IrO.sub.2.2H.sub.2O (Alfa Aesar)), a 5 wt
% nafion solution (DuPont Co.), deionized water and isopropyl
alcohol with a weight ratio of 7:60:42:168 during the spraying
process. At this time, an IrO.sub.2 loading amount and a nafion
content were controlled to 1.0 mg/cm.sup.2 and 30 wt %,
respectively.
[0101] (2) Preparation of HT-PEMWE Single Cell
[0102] A single cell was fabricated by assembling a cathode end
plate, a graphite anode plate, a hydrogen electrode (exposed area:
6.25 cm.sup.2), a nafion membrane (NR-212, Dupont), an oxygen
electrode, an Au/Ti bipolar plate (BP), and an anode end plate. The
hydrogen electrode was prepared by spraying a catalyst ink (46.5 wt
% Pt/C (TKK)), a 5 wt % nafion solution, deionized water and
isopropyl alcohol into commercial carbon paper (39BC, SGL carbon).
At this time, the Pt loading amount and the nafion content were
controlled to 0.4 mg/cm.sup.2 and 30 wt %, respectively.
[0103] Measurement Equipment
[0104] Surface morphology and IrO.sub.2 distribution at the
substrate were examined with field emission scanning electron
microscopy (Inspect F50, Field emission Inc.) and electron probe
micro analysis (EPMA, JXA-8500F, JEOL). Focused ion beam (FIB, Nova
600, FEI) was used to observe the cross-sections of prepared
samples. Before FIB was used, the Pt layer was preliminarily
deposited onto the sample using a sputtering process for protecting
the inner sample. The X-ray photoelectron spectroscopy (XPS, PHI
5000 VersaProbe, Ulvac-PHI Ltd.) using Al K.alpha. (1486.6 eV)
radiation and inductively coupled plasma mass spectroscopy (ICP-MS,
iCAP 6300 series, Thermo Ltd.) was used to analyze the oxidation
state of Ir and the adsorption amount of IrO.sub.2.
[0105] The corrosion behavior of the diffusion layer (DL) was
examined using linear sweep voltammetry analysis with a scan rate
of 1 mV/s and a potential range of -0.8 to 1.0 V.sub.SCE, and an
analysis was performed in a 0.5 M H.sub.2SO.sub.4 solution at
25.degree. C.
[0106] Furthermore, for the single cell, with inflowing deionized
water (15 mL/min) into the anode, iV curve and EIS spectrum were
obtained by using potentiostat (HCP-803, biologic) at 120.degree.
C. and 2.5 bar (for both anode and cathode). For obtaining iV
curves, the voltages were sequentially applied from 1.35 V to 2.0
Vat a predetermined voltage interval of 0.05 V. For each voltage, a
mean current density was obtained for 60 sec. The EIS measurement
was conducted at a DC potential of 1.72 V and an AC frequency of 50
kHz to 50 mHz. A degradation experiment was carried out with a
constant voltage of 1.72 V (voltage efficiency: 85%) at 150.degree.
C. and 2.5 bar.
[0107] Results and Discussion
[0108] Confirmation of Current Flow of Anode According to
Deposition Time and Deposition Amount
[0109] An anodic electrodeposition process of IrO.sub.2 on a Ti
mesh was conducted under a condition of E.sub.dep of 0.5 to 0.9
V.sub.SCE, and the current profile during IrO.sub.2
electrodeposition is shown in FIG. 3. Referring to FIG. 3, the
current density was quite low at 0.5 V.sub.SCE and 0.6 V.sub.SCE.
However, when E.sub.dep was increased to 0.7 to 0.9 V.sub.SCE, the
current became observable, and this current flow is determined to
be a result of the oxidation reaction of [Ir(COO).sub.2
(OH).sub.4].sup.2- complex with IrO.sub.2. At 0.7 to 0.9 V.sub.SCE,
the current sharply grew at the initial stage of electrodeposition,
but the growth became slow down after few seconds, and the initial
increase in current is determined to be because the OER was more
efficiently catalyzed by deposited e-IrO.sub.2 than the inner Ti
substrate.
[0110] The surface morphologies and the corresponding EPMA mapping
for electrodes with various E.sub.dep (0.6, 0.7, 0.8, and 0.9
V.sub.SCE, t.sub.dep: 5 min) are shown in FIGS. 4A to 4E (FIG. 4A:
pristine Ti mesh, and FIGS. 4B to 4E: iridium oxide-deposited Ti
meshes under conditions of 0.6 V.sub.SCE (FIG. 4B), 0.7 V.sub.SCE
(FIG. 4C), 0.8 V.sub.SCE(FIG. 4D), and 0.9 V.sub.SCE (FIG. 4E) for
5 min, respectively). Referring to this, as in FIG. 3, it could be
confirmed that even at E.sub.dep of 0.6 V.sub.SCE where the current
density was quite low, a small amount of IrO.sub.2 was precipitated
on the Ti surface. Meanwhile, when E.sub.dep was increased to 0.7
V.sub.SCE, the deposition amount was increased, and the Ti surface
seemed to be fully covered with IrO.sub.2 film without any
noticeable cracks (FIG. 4C), and at 0.8 V.sub.SCE and 0.9
V.sub.SCE, IrO.sub.2 is still uniformly applied on the Ti surface
and an electrode detachment was not found on the e-IrO.sub.2/CP
electrode at high E.sub.dep (E.sub.dep: 0.8 and 0.9 V.sub.SCE).
From the absence of IrO.sub.2 separation at high E.sub.dep, it
could be confirmed that there was good adhesion between IrO.sub.2
and Ti surface.
[0111] Meanwhile, though the higher E.sub.dep led to higher
deposition current density (FIG. 3), the deposition amount of
IrO.sub.2 was not varied between 0.7 and 0.9 V.sub.SCE. As
described above, the current efficiency for IrO.sub.2 deposition at
0.8 V.sub.SCE and 0.9 V.sub.SCE was low, and therefore, the
experiment was carried out only at 0.7 V.sub.SCE.
[0112] The oxidation state of electrodeposited Ir thin film on Ti
(E.sub.dep=0.7 V.sub.SCE, t.sub.dep=5 min) was confirmed with XPS
analysis. In FIG. 5, two asymmetrical peaks related to Ir4f were
observed at 65.1 eV (4f.sub.5/2) and 62.2 eV (4f.sub.7/2), which
also seemed to be fitted with doublets. When each peak was
deconvoluted into two peaks, the peaks with lower binding energies
at 65.10 eV (4f.sub.5/2) and 62.10 eV (4f.sub.7/2) could be
assigned to an Ir.sup.4+ state, indicating the presence of
IrO.sub.2. The higher binding energy peaks at 66.55 eV (4f.sub.5/2)
and 63.55 eV (4f.sub.7/2) can be originated from the unscreened
components of the Ir.sup.4+ state, and thus, it could be confirmed
that the surface of precipitates was mainly composed of
IrO.sub.2.
[0113] The change in IrO.sub.2 thin film according to the change in
time at E.sub.dep of 0.7 V.sub.SCE is shown in FIGS. 6A to 6E, and
even at very short t.sub.dep of 1 min (FIG. 6A), the film phase was
formed on Ti phase indicative of high nucleation density of
IrO.sub.2 followed by rapid film formation. Until t.sub.dep of 5
min (FIGS. 6B and 6C), the IrO.sub.2 thin film uniformly grew on
the Ti surface without any formation of surface defects. However,
when t.sub.dep was further increased to 10 min and 20 min (FIGS. 6D
and 6E), the crack and IrO.sub.2 agglomerates began to be formed.
The formation is determined to probably result from vigorous OER
that hindered uniform growth of IrO.sub.2 thin film, and the crack
formation at high t.sub.dep was similarly found for the
e-IrO.sub.2/CP electrode. The loading amount of IrO.sub.2 as a
function of t.sub.dep (E.sub.dep=0.7 V.sub.SCE) is shown in FIG.
7A. The loading amount of IrO.sub.2 was roughly proportional to
t.sub.dep with a slope of about 0.141 mg/cm.sup.2min, indicating a
current efficiency of about 15%. The low current efficiency
suggested that the majority of charge during IrO.sub.2 deposition
was consumed by OER (see FIG. 3). FIG. 7B is a cross-sectional view
of an e-IrO.sub.2/Ti electrode (E.sub.dep=0.7 V.sub.SCE,
t.sub.dep=5 min) analyzed with FIB-SEM. The e-IrO.sub.2 film with a
uniform thickness (183 (.+-.26) nm) was seen on the entire surface
of Ti, suggesting that the e-IrO.sub.2 could physically block the
inner Ti from the external oxidative environment. The thickness was
used to evaluate the volume of IrO.sub.2 on Ti (.pi.n (Dt+t.sup.2),
D=diameter of fiber, t=thickness of IrO.sub.2, and n=number density
of fiber). The measured density of IrO.sub.2 was 8.63 g/cm.sup.3,
which was slightly lower than that of bulk IrO.sub.2 (11.66
g/cm.sup.3). The lower density of e-IrO.sub.2 than that of bulk
IrO.sub.2 was because the e-IrO.sub.2 was basically in amorphous
state (p<11.66 g/cm.sup.3). However, compared to density values
of e-IrO.sub.2 (.about.2 mg/cm.sup.3) found in other previous
studies, the density of IrO.sub.2 in the present study was
relatively high, implying the low density of inner-pore that was
essential for effective corrosion prevention.
[0114] Single Cell Performance Experiment
[0115] The performance of a single cell including an e-IrO.sub.2/Ti
electrode (E.sub.dep=0.7 V.sub.SCE, t.sub.dep=1 to 20 min) was
evaluated at 120.degree. C. and 2.5 bar (FIG. 8A). It could be
confirmed that the current density at given potential was dependent
upon not only the catalyst loading amount but also the electrode
morphology that varied with t.sub.dep. When t.sub.dep was increased
from 1 min to 5 min, the current density at 1.6 V was gradually
increased from 0.07 A/cm.sup.2 to 0.96 A/cm.sup.2. However, during
that period, the catalyst loading amount was raised up from 0.01 to
0.40 mg/cm.sup.2, while no significant change in electrode
morphology was seen. In addition, when t.sub.dep was further
increased up to 10 min, the current density at 1.6 V was suddenly
dropped to 0.67 A/cm.sup.2 despite the increase in catalyst loading
amount to 1.05 mg/cm.sup.2 (FIG. 7B). The further increase in
t.sub.dep to 20 min resulted in IrO.sub.2 loading amount of 2.70
mg/cm.sup.2, but rather decreased the current density to 0.38
A/cm.sup.2. The decrease in current density is determined to be
because cracks are generated on the electrode surface and IrO.sub.2
is agglomerated (FIGS. 6D and 6E).
[0116] An overpotential analysis was carried out in order to
explain the sudden drop of electrode efficiency at higher t.sub.dep
(10 min and 20 min)(FIG. 8B). As shown, it could be confirmed that
the ohmic overpotential (.eta..sub.ohm) was gradually increased
when t.sub.dep was increased from 5 min (0.105 V) to 10 min (0.168
V) and to 20 min (0.267 V), and the mass transfer overpotential
(.eta..sub.mass) was very small when t.sub.dep=5 and 10 min but was
suddenly increased up to 0.185 V at t.sub.dep=20 min. Meanwhile,
the kinetic overpotential (.eta..sub.kin) that depended on catalyst
activity was changed to only a small extent (0.26 to 0.28 V).
Accordingly, it seems that the performance drop at high t.sub.dep
(10 and 20 min) was primarily ascribed to the ohmic and mass
transfer losses. A possible explanation for the increase in
.eta..sub.ohm is the retarded electron conduction due to the
oxidation of Ti during anodic IrO.sub.2 electrodeposition. During
the IrO.sub.2 electrodeposition process, the Ti substrate was
simultaneously oxidized into non-conductive TiO.sub.2, and this
process might be prompted at the cracked sites that were found when
t.sub.dep=10 and 20 min (FIGS. 6D and 6E). As TiO.sub.2 became
thicker, the electron conductivity on Ti was gradually decreased,
leading to the increase in norm. Meanwhile, a sudden increase in
.eta..sub.mass at t.sub.dep=20 min was possibly due to the
difficulties of 02 transfer at cracks on the IrO.sub.2 surface, but
further experiments are required in order to clarify the precise
reason.
[0117] The current densities of e-IrO.sub.2/Ti electrodes
(E.sub.dep=0.7 V.sub.SCE) at 1.6 V were compared with those of the
apparatuses where the operating temperature was 110 to 130.degree.
C., which are described in B.-S. Lee, H.-Y. Park, I. Choi, M. K.
Cho, H.-J. Kim, S. J. Yoo, D. Henkensmeier, J. Y. Kim, S. W. Nam,
S. Park, K.-Y. Lee, J. H. Jang, J. Power Sources 309 (2016)
127-134. (Non-patent document 1), and the like. (Non-patent
documents 3 to 12) (FIG. 9A). At the documents, the catalyst
loading amount ranged from 1 to 5 mg/cm.sup.2, which indicated 0.3
to 1.25 A/cm.sup.2 at 1.6 V. For comparison, the e-IrO.sub.2
electrode (E.sub.dep=0.7 V.sub.SCE, t.sub.dep=5 min)(loading
amount: 0.4 mg/cm.sup.2) exhibited a current density of 0.96
A/cm.sup.2, which was higher than those of most publications
(Non-Patent Documents 4, 5, 6, 8, 9, 10, and 12), and comparable to
those of a few publications at the highest levels (Non-Patent
Documents 3, 7, and 11). The current density at 1.6 V was divided
by the anode loading amount, and the resultant mass activity value
is shown in FIG. 9B. Under an optimized condition (E.sub.dep=0.7
V.sub.SCE, t.sub.dep=5 min), the e-IrO.sub.2 electrode showed a
mass activity of 2.43 A/mg, which is about 4-fold higher than the
highest value reported in the Non-Patent Documents (Non-Patent
Document 11, 0.62 A/mg). This showed the excellent catalyst
utilization and cost efficiency of e-IrO.sub.2/Ti electrodes.
[0118] As mentioned previously, the Ti diffusion layer may be
oxidized during cell operation and the oxidation may be prevented
by coating a noble metal such as Pt or Au on the Ti diffusion
layer. The electrodeposited IrO.sub.2 catalyst is capable of
protecting Ti from oxidation, requiring no more additional
oxidation prevention films. It could be confirmed that the
stability of Ti after IrO.sub.2 electrodeposition was tested by a
polarization analysis in 0.5 M H.sub.2SO.sub.4 (FIG. 10), pristine
Ti (t.sub.dep=0 min) exhibited an open circuit potential (OCP) of
-0.18 V.sub.SCE in consistency with the former work, and on
sweeping potential to positive direction, a small current peak was
formed at 0.02 V.sub.SCE, and the current was stabilized at 0.1 to
1.0 V.sub.SCE. This behavior was a general passivation behavior of
Ti and the potential where the current peak was formed is referred
to as a passivation potential, and physically means a spot where
the passivation begins. Meanwhile, the current peak, which is also
referred to as the critical current density (Ic), has been regarded
as an indicator showing the stability of a passivation film, and
the lower current density means a better stability. When IrO.sub.2
was deposited, the OCP shifted in a positive direction, and Ic was
changed from 31.76 pA/cm.sup.2 (0 min) to 5.66 pA/cm.sup.2 (5 min),
clearly indicating the improvement of oxidation resistance
property.
[0119] Furthermore, the durability of the e-IrO.sub.2/Ti electrode
could be determined by an aging experiment performed at 1.72 V
(voltage efficiency: 85%), 120.degree. C., and 2.5 bar, and for
comparison, the durabilities of e-IrO.sub.2/CP and s-IrO.sub.2/Ti
electrodes were also observed under the same operating conditions
(FIG. 11).
[0120] As shown in FIG. 11, the e-IrO.sub.2/Ti exhibited the lowest
degradation rate of 1.5 mA/cm.sup.2-hr (0.11%/hr), and this value
is quite lower than that of e-IrO.sub.2/CP (49.4 mA/cm.sup.2-hr
(1.74%/hr)), indicating the significance of the diffusion layer.
More importantly, e-IrO.sub.2/Ti was more stable than
s-IrO.sub.2/Ti, a typical form of the oxygen electrode. The
s-IrO.sub.2/Ti exhibits a degradation rate of 9.6 mA/cm.sup.2-hr
(1.20%/hr) for initial 25 hr, and exhibits a degradation rate of
1.8 mA/cm.sup.2-hr (0.22%/hr) for 25 to 150 hr. This is due to the
improved stability of the Ti diffusion layer from
oxidation/corrosion, as shown in FIG. 10, which is compared to the
rapid degradation of spray-type electrode at high temperature.
[0121] The stability of the e-IrO.sub.2/Ti electrode is at a level
which is about 2-fold better than the electrode stability reported
in Non-Patent Document 13 (degradation rate of 0.75 mA/cm.sup.2-hr
(0.21%/hr) at 110.degree. C. and 2.78 mA/cm.sup.2-hr (0.23%/hr) at
150.degree. C.). The electrode degradation is determined to
partially due to the chemical deformation of the nafion membrane at
high temperature/high pressure conditions, but considering the
experimental results, the electrode degradation is determined to be
associated with the oxidation of the Ti diffusion layer, and this
is determined to be capable of being prevented by the coating of
the corrosion-resistive film. Accordingly, considering the catalyst
activity, stability, economic efficiency, and the like, it could be
confirmed once again that e-IrO.sub.2/Ti is an electrode for
HT-PEMWE application.
[0122] The Examples of the present disclosure previously described
should not be interpreted to limit the technical spirit of the
present disclosure. The scope of the present disclosure to be
protected is limited only by the matters described in the claims,
and those skilled in the art of the present disclosure can improve
and change the technical spirit of the present disclosure in
various forms. Therefore, such improvements and changes would fall
within the scope of the present disclosure to be protected as long
as they are obvious to those skilled in the art.
* * * * *